1. Field of the Invention
This invention generally relates to a thin film resistance memory device, for non-volatile memory arrays, and more particularly, to a bipolar programmable memory cell having an asymmetric area.
2. Description of the Related Art
State of the art resistor random access memories (RRAMs) are often made from a colossal magnetoresistance (CMR) memory film and, more typically, from a Pr0.3Ca0.7MnO3 (PCMO) type of CMR material. The CMR material can be said to have a nonvolatile nature, as the resistance of the CMR material remains constant under most circumstances. However, when a high electric field induces current flow through the CMR material, a change in the CMR resistance can result. During narrow-pulse programming, the resistivity of the memory resistor near an electrode changes. Experimental data shows that the resistivity of the CMR material near the cathode increases while that at the anode decreases. During the erase process the pulse polarity is reversed. That is, the designation of cathode and anode are reversed. Then, the resistivity of the material near the cathode decreases, and the resistivity near the anode increases.
FIG. 1 is a diagram of a symmetrical CMR film memory cell (prior art). The device is called symmetric because it has a uniform area along any cross-section of the CMR film thickness. The memory cell can be written to high-resistance state using either positive or negative narrow pulse, and reset to low-resistance state using a long-width electrical pulse. Other memory cells (not shown) may be written to high-resistance state and erased to low-resistance state using a narrow negative pulse and a narrow positive pulse, respectively. A memory device that is responsive to only one type of programming, either bipolar or uni-polar, has a necessarily limited usefulness, and is dependent upon system specifications and available power supplies.
Thus, some systems are designed for bipolar programming, whereas others are designed for uni-polar programming, depending upon the type of CMR memory cell being used. This uncertainty in the design of the memory cells necessarily increases production costs. Some CMR film memory cell designs are made bipolar programmable by manipulating the composition of the film along the thickness of the RRAM resistor. As the memory cell size is reduced, the thickness of the memory resistor thin film is also reduced. However, CMR film thickness can be a difficult variable to control.
It would be advantageous if a CMR memory cell could be programmed using bipolar, as well a uni-polar pulses.
It would be advantageous if a process for fabricating a CMR memory cell that is responsive to either bipolar or unipolar programming, could be made scalable for processes using increasing smaller feature sizes.